This invention relates to optical recording systems. In particular, this invention relates to the focusing detection systems to be used with optical media recording systems.
It is well known that optical recording systems require a very precisely focused condition of the light beam through an objective lens onto the recording media. The light beam focused on the media is, typically, reflected back through the objective lens and used in the system to read information on the media. The reflected light beam is typically divided into additional parts including a part, typically, used for focus detection information and to operate a focusing control system for a movable objective lens.
In optical recording technology, various focusing and light beam handling systems are known. In U.S. Pat. No. 3,974,327 a schematic diagram is shown of a light beam being returned directly from an objective lens to a light detector array from which a particular focusing system operates. A mirror which is partially reflecting and partially transparent may be used to control the splitting of the light beam into various elements for various purposes. U.S. Pat. No. 4,123,652 shows the use of partially reflecting and partially transparent mirrors or beam splitters to divide a light beam into various components in connection with both an information reading detector for one beam element and a focusing light detector array for another beam element. U.S. Pat. No. 4,123,652 shows what is known as the astigmatic focusing system whereby an image is distorted in a first direction in a first out of focus condition with respect to the recording media and in a second direction in a second out of focus condition with respect to the recording media. The two out of focus conditions with respect to the recording media are those involving the objective lens being too close to the media and too far from the media. It is noted that FIG. 7 of that patent shows a cylindrical lens element in connection with the focusing of a light beam on the light detector array. However, this patent is related to astigmatic focusing systems rather than partial beam focusing systems.
Also known to applicant are U.S. Pat. Nos. 3,969,576 and 4,143,402 relating to partial beam focusing systems.
In U.S. Pat. No. 3,969,576, separate light detecting elements are arranged to receive a portion of a light beam for use in reading information and another light detecting element is disposed at a different location to receive a light beam for purposes of operating a focusing system. The focusing beam is referred to in that patent as an auxiliary beam while the beam from which information is read is referred to as the read beam. A radiation absorbing knife is disposed between the radiation source and a Wollaston prism to block a portion of the auxiliary beam but which is so disposed with respect to the source and the prism that none of the read beam is blocked. As explained in the patent, the auxiliary beam is returned to the focusing light detector array as only a partial beam. In the focused condition, a comparatively small focused point of light exists on a pair of light detector elements. In one out of focus condition, a partial light beam, sometimes referred to as a half-moon, is focused on one-half or one element of the light detector array and in the other out of focus condition a partial light beam image or half-moon is shown on the other light detector element of the light detector array. This patent therefore represents the basic half-moon or partial light beam focusing system which is now comparatively well known.
U.S. Pat. No. 4,143,402 shows another half-moon focusing system to which the light from a source is only partially returned to a focusing detector system and in which the half-moon light spot returned to the focusing source occurs on one half or one portion of the light detector array for one out of focus condition and on the other portion of the light detector array for the other out of focus condition. In that patent, a prism is shown which reflects one-half of a portion of the light beam returned from the media to the focusing detector array while not affecting the other half of the light beam. The prism is disposed in such a fashion that it does not use all of the reflected light from the media for focusing on the detector array but allows a portion of the original beam to pass to an information reading detector array.
Both of the aforementioned partial beam focusing detection system patents show a means for forming a partial light beam which is also disposed in the path from the light source to the media and is not disposed so as to only affect light being reflected from the media. In systems adapted for reading from the media only, these systems are not as critical as to light lost in the partial beam system.
However, for systems in which data is to be both read and written, a partial beam system or system in which the light beam travelling towards the media may be reduced in strength have a critical disadvantage. In systems in which the media is to be written, it is important to conserve the power of the writing beam so that only a minimum power writing source may be provided. If the systems shown in the aforementioned partial beam patents were to be used in a reading and writing system, they would suffer at least one of several possible disadvantages such that there would be less power in the writing beam, larger spot size on the media than desired because of necessary additional optical elements interposed in the writing beam or comparatively complex or large sized lenses; all as compared to a system without such partial beam elements interposed in the writing beam.
Therefore, in systems in which focus is to be maintained during writing on the media, it is clear that it is necessary to provide a focus detection system which does not require the provision of any additional optical elements which may interfere with the writing beam. Also, the aforementioned patents result in an extremely critical physical relationship between the partial beam creating device, the knife in the one patent or the prism in the other patent, and the light detecting element. If there should be any misalignment between the focusing light detector array and the prism or the knife, then the focusing system will not work as intended. It is therefore desirable to provide a focusing system having a somewhat less critical relationship between the partial beam transmitting means and the focusing light detector array.
With respect to the astigmatic focusing system as shown in U.S. Pat. No. 4,123,652, it is noted that the astigmatic focusing system requires two cylinder lenses or one cylinder lens and one spherical lens or some other combination of multiple optical elements in order to create the astigmatic focusing pattern. Therefore, these additional optical elements create additional optical complexity or else cause loss of light between the media and the focusing detector array.
It is clear that it would be desirable to provide a focusing detection system having some of the advantages of both the astigmatic focusing system and of the knife edge focus detecting system.